Oxygen demand is an important parameter for determining the effect of organic pollutants on receiving water. As microorganisms in the environment consume these materials, oxygen is depleted from the water. This can have an adverse effect on fish and plant life.
There are three main methods of measuring oxygen demand: directly, by biochemical oxygen demand (BOD) and/or chemical oxygen demand (COD), and indirectly by total organic carbon (TOC) procedures. BOD, because it uses microorganisms for oxidation, gives the closest picture of the biological processes occurring in a stream. However, results are not available for five days, and the BOD test is inadequate as an indicator of organic pollution when used with industrial wastewater containing toxic materials which poison the microorganisms and render them unable to oxidize wastes.
Unlike BOD, the two other methods do not use biological processes and are therefore faster. A strong oxidizing agent or combustion technique is used under controlled conditions in the TOC method to measure the total amount of organic material in a sample. The results obtained may not be as accurate as the results reached through the COD or BOD method in predicting environmental oxygen demand because oxygen demands may differ between compounds with the same number of organic carbons in their structures. The difference in oxygen demand between two compounds containing the same amount of organic carbon can be seen in the following equations showing the oxidation of oxalic acid and ethanol: EQU oxalic acid: C.sub.2 H.sub.2 O.sub.4 + 1/2O.sub.2 .fwdarw.2CO.sub.2 +H.sub.2 O EQU ethanol: C.sub.2 H.sub.5 OH+3O.sub.2 .fwdarw.2CO.sub.2 +3H.sub.2 O
Each molecule of ethanol uses up six times as much oxygen as an equivalent amount of oxalic acid and thus would have a much greater effect on the dissolved oxygen present in a receiving water. Estimating environmental oxygen demand (as with BOD and COD) requires complete oxidation of carbon and hydrogen present in the organic matter. Thus, while TOC is a more direct expression of total organic content than BOD or COD, it does not provide the same kind of information. An empirical relationship can exist between TOC, BOD and COD, but the specific relationship must be established for a specific set of sample conditions.
Currently, the COD test has a fairly specific and universal meaning: the oxygen equivalent of the amount of organic matter oxidizable by potassium dichromate in a 50% sulfuric acid solution. Generally, a silver compound is added as a catalyst to promote the oxidation of certain classes of organics. A mercuric (or other) compound may also be added to reduce interference from the oxidation of chloride ions by the dichromate which will give false high COD readings. The end products of organic oxidations are carbon dioxide and water.
After the oxidation step is completed, the amount of dichromate consumed is determined either titrimetrically or colorimetrically. Either the amount of dichromate reduced (Chromium III) or the amount of unreacted dichromate (Chromium VI) remaining can be measured. If the latter method and colorimetry are chosen, the analyst must know the precise amount of dichromate added and be able to set the instrument wavelength very accurately since readings are routinely taken on the "shoulder" of the Chromium VI absorbance peak. Wavelength settings must be reproduced precisely in order to avoid errors when using a previously generated calibration curve.
Dichromate was first used in the COD test over 50 years ago. Before that time, potassium permanganate was the oxidant of choice. Analysts have tried many other reagents such as potassium persulfate, cerium sulfate, potassium iodate and oxygen itself. Generally these other oxidants have not been satisfactory.
While oxidation of organic materials by dichromate in sulfuric acid has been the method of choice for perhaps 50 years, it too is not without limitations. However, through careful research, many COD method limitations have been overcome or significantly reduced. For example, incomplete oxidation of aliphatic hydrocarbons, organic acids and alcohols has been improved by using silver ions as a catalyst, although heterocyclic compounds containing nitrogen still present difficulties. Interference from oxidation of chloride can be reduced by the addition of mercuric (or other) ions to the sample. A simple mathematic correction is not sufficient because chloride is not always oxidized quantitatively. In addition, the presence of chloride enhances the interference of ammonia nitrogen. Mercuric (or other) ions form a soluble complex with chloride which largely eliminates its interference up to 2000 mg/L. An alternative method for sea water and brines is to collect chlorine produced by oxidation in a potassium iodide solution and titrate to determine the correction factor. The addition of mercury is undesirable from a pollution standpoint; however, when mercury is used, it is required by law to be recovered and recycled.
Although nitrite is seldom present at high enough levels to cause significant interference, it can be eliminated by adding sulfamic acid. If high concentrations of other reducing or oxidizing agents are present, a separate analysis and correction may be necessary in all methods.
The relatively long two-hour digestion time now in use can be reduced if caution is observed. Many types of wastes are digested completely in 30 minutes or less at 150.degree. C., the normal operating temperature. The time of complete digestion can be recognized by an experienced operator or (more efficiently) by using a colorimetric reading with the micro method explained subsequently. In this approach, many consecutive readings taken on a single sample are used to determine when there is no further change in dichromate concentration due to reduction. At this point, the oxidation would be considered complete and the final determination could be performed. The escape of volatile organics, cumbersome equipment and large amounts of expensive silver compounds called for in the EPA and Standard Methods Open Reflux procedures have all been eliminated by the micro method. Sealed digestion containers prevent the escape of volatiles and eliminate the need for condensers. When the water sample is added to the acid dichromate solution, it forms a layer on top due to the difference in density between the water and the acid. When the water and acid mix, the heat generated can drive off some of the volatiles. (Some volatiles are not driven off until higher temperatures are reached during the heating process.) Loss of volatiles is prevented by having the cap screwed onto the vial when the mixing and subsequent heating occurs in the micro method.
The chemistry of COD digestions when using dichromate in sulfuric acid is well known. The carbon compounds are converted to carbon dioxide while hydrogen liberated from hydrocarbons is converted to water. Other elements may also be oxidized. There is some disagreement, however, as to what "complete" oxidation is, especially when nitrogen-containing compounds are present. The main reaction is illustrated in Equation 1 with potassium acid phthalate (KHP) used as an example. EQU 2KC.sub.8 H.sub.5 O.sub.4 +10K.sub.2 Cr.sub.2 O.sub.7 +41H.sub.2 SO.sub.4 .fwdarw.16CO.sub.2 +46H.sub.2 O+10Cr.sub.2 (SO.sub.4).sub.3 +11K.sub.2 SO.sub.4 ( 1)
Because each molecule of K.sub.2 Cr.sub.2 O.sub.7 has the same oxidizing power as 1.5 molecules of O.sub.2, the equivalent reaction is: EQU 2KC.sub.8 H.sub.5 O.sub.4 +15O.sub.2 +H.sub.2 SO.sub.4 .fwdarw.16CO.sub.2 +6H.sub.2 O+K.sub.2 SO.sub.4 ( 2)
Thus, two (2) molecules of potassium acid phthalate (KHP) consume 15 molecules of oxygen. On a weight basis, the theoretical COD for one (1) milligram (mg) of KHP is 1.175 mg of oxygen.
In spite of the approved technology of dichromate in sulfuric acid testing as above described, there remain several problems with the dichromate COD test. In the first instance, Chromium VI (dichromate) is a known carcinogen, and therefore those making up the samples are exposed to a potential risk from Chromium VI. Secondly, when Chromium VI is reduced to Chromium III there is a color change from a yellowish-orange to green. Thus, the dichromate analysis includes a two-color system that does not lend itself to visual analysis and can require very accurate instrumental wavelength settings. Third, the dichromate reagent is photosensitive, and the results can be affected by prolonged exposure to light. Fourth, to properly do the analysis, as earlier mentioned, silver ion is needed as a catalyst and mercuric (or other) ion is needed to effect the removal of chloride. Obviously, since one is measuring the amount of organic material in the aqueous sample, any dichromate used in oxidation of inorganics such as chloride causes an error in the COD analysis results. Thus mercuric (or other) ion is commonly added to rid the system of chloride interference. Silver is expensive and heavy metals such as silver and mercury need to be reclaimed and recycled due to possible environmental problems.
It can therefore be seen that there is a continuing need for the development of new and accurate COD tests. This invention has as its primary objective the fulfillment of the need for the development of a new test which does not employ a dichromate in sulfuric acid analysis.
Another objective of the present invention is to provide a COD test which is based upon a non-photosensitive, non-carcinogenic, stable oxidizing agent.
A further objective of the present invention is to provide a Manganese III ion based COD test system.
An even further objective of the present invention is to provide a system which employs a single color change, making the test results easy to read visually using a color comparator device or instrumentally using a colorimeter or spectrophotometer on either an intermittent or continuous basis.
Still another objective of the present invention is to prepare a COD test system which is inexpensive to produce and easy to make while minimizing the risks involved in handling potentially hazardous chemicals.
The method and means of accomplishing each of the above objectives as well as others will become apparent from the detailed description of the invention which follows hereinafter.